Nuclear reactors using ordinary water under pressure for producing electrical power include control clusters each of which comprises a plurality of neutron-absorbing rods. These rods serve to modulate the power provided by the reactor as a function of the degree to which they are inserted into the reactor core. Each control cluster is provided with an operating mechanism which is placed on the reactor vessel and which serves to displace the cluster vertically relative to the vessel. The operating mechanism associated with each cluster is of the actuator type and provides step-by-step displacement. It comprises three electromagnetic windings selectively powered by a corresponding number of feed bridges, referred to as power bridges. These bridges are collected together in static power assemblies which are controlled from a control desk via a control logic circuit which acts on the feed bridges via logic interfaces placed in the static power assemblies. For safety reasons, the equipment, and in particular its static power assemblies, is provided in the form of a plurality of units which are independent from one another.
In some of the power-producing nuclear reactors in service, the control logic circuit and the logic interfaces are provided in hard-wired form, whereas in more modern reactors the control logic circuits and the logic interfaces are provided in programmed form.
Hard-wired logic circuits do not have the same flexibility as programmed logic circuits when operating changes become necessary. When modifying, maintaining, or repairing hard-wired logic circuits, the equipment associated therewith needs to be taken out of service for on-site manual operations which take periods of time that bear no comparison with the time required for such circuit card swapping as may be necessary in equivalent programmed logic circuits.
This is particularly important in power-producing nuclear reactors where shut-downs should be kept to a minimum, both in duration and in quantity.
The necessary operations are therefore to be performed in as short a time as possible during the obligatory shut-down periods programmed for such reactors.